The present invention relates to a process for recovering metals from tar. In particular, the present invention relates to a process for recovering Group VIII noble metals, which have been used as catalysts in carbonylation processes, from tar produced as byproducts in such processes.
The use of Group VIII noble metals to catalyse a range of carbonylation processes is now well established. In particular liquid phase homogenous carbonylation processes using rhodium catalysts have been shown to be capable of commercial exploitation. Typical examples of such processes include the carbonylation of methanol to form acetic acid, the hydroformylation of olefins to produce high molecular weight aldehydes and alcohols and the carbonylation of methyl acetate to produce acetic anhydride or ethylidene diacetate.
A potential problem, however, exists in the use of such Group VIII noble metals as catalysts on a commerical scale, namely the possibility of catalyst loss. It is extremely desirable to minimise losses if possible because Group VIII noble metals are extremely expensive and significant losses can easily make the operation of commercial plants uneconomic.
One potential source of catalyst loss is in any tars which are produced as a side product during the carbonylation reaction. For example in the rhodium catalysed carbonylation of methyl acetate to acetic anhydride, it is known that tars are produced in considerable quantities and that such tars tend to absorb the rhodium catalyst and any associated catalyst promoters and copromoters thereby depleting the reaction mixture of valuable active catalyst. In such cases it is necessary to include as part of the total process, a processing step which not only removes the tar from the reaction mixture, but also frees the catalyst components from the tar and converts them into a form suitable for recycling.
A tar removal/catalyst recovery process of the type contemplated above has been disclosed in PCT International application 82/01829. This process, for use with the lithium promoted, rhodium catalysed carbonylation of methyl acetate to acetic anhydride, comprises; removing by distillation, any volatile material from a process stream containing tar catalyst and promoter; mixing the tar/catalyst/promoter mixture with methyl iodide and finally contacting the tar/methyl iodide/catalyst/promoter mixture with aqueous hydrogen iodide under conditions which effect the extraction of the catalyst and promoter into the aqueous hydrogen iodide. Since the methyl iodide/tar mixture and the aqueous hydrogen iodide are immiscible they can easily be separated after the extraction. Once the separation is complete the rhodium catalyst and lithium promoter can be recovered from the aqueous hydrogen iodide. The methyl iodide/tar mixture is fed after the separation to a distillation unit where the methyl iodide is removed overhead and recycled whilst the tar is removed at the bottom and burnt.
An alternative process has now been discovered which is applicable to the recovery of any Group VIII noble metal catalyst from a tar. The process involves electrodialysing the Group VIII noble metal catalyst from a liquid medium containing the tar directly into a stream comprising a solvent.